Nowitzki had an MRI exam on his right knee Tuesday morning that revealed a sprain, the team announced. The Mavericks listed him as day to day.
Nowitzki told ESPN.com before undergoing the MRI that he didn't believe the knee sprain -- suffered in Monday night's victory at Oklahoma City when he landed awkwardly -- was serious.
"I'm not worried," Nowitzki said.
The All-Star forward prides himself on his durability and has missed just 32 games to injury in his 13-season career, the third-lowest total among active players with at least 10 years of experience, behind Portland's Andre Miller (six) and Mavs teammate Jason Terry (25).
One source said Nowitzki remains hopeful of playing Thursday against the Spurs, but Dallas likely will be cautious with the injury, because it involves a knee as opposed to one of his ankles, after years of seeing Nowitzki recover from ankle injuries ahead of schedule.
Nine ankle sprains of varying degrees since 2001-02 have sidelined Nowitzki for just 16 games.
X-rays taken Monday night on Nowitzki's knee were negative, but he told reporters in Oklahoma City that the knee had been bothering him for about two weeks.
"So maybe that's why it gave in a little bit," Nowitzki said. "Hopefully it's just a strain and hopefully I'll be back in action soon."
Nowitzki left Monday's game at the 9:10 mark of the second quarter after his momentum from a fallaway jumper caused him to stumble backward to the floor. He was fouled on the arm by Thunder center Serge Ibaka, but Nowitzki lost his footing on the way down.
"It was just a scary play," Nowitzki said. "The foul was on my arm so it was just on the landing. I landed awkward and strained it more than I guess it already was. It [the knee] kind of gave in."
The 32-year-old is averaging 24.5 points and 7.6 rebounds and shooting 54.5 percent from the field, winning raves around the league for playing as well as he did in his MVP season of 2006-07.
Nowitzki has mostly avoided knee injuries, with the exception of a sprained left knee that forced him out of the 2003 Western Conference finals for the final three games of a series that the Spurs eventually won in six.
A dispute about letting Nowitzki play through the injury is generally regarded as the issue that doomed the relationship between Mavs owner Mark Cuban and former Mavs coach Don Nelson, who was determined to hold Nowitzki out.
"I'll take an ankle sprain over this any day just because this is a weird injury," Nowitzki said Monday night. "I never had it, so from a rolled ankle I probably would have walked it off and came right back out there. We'll look at it and see what it is tomorrow and [take it] kind of day to day from this point."
Marc Stein covers the NBA for ESPN.com and is a frequent contributor to ESPNDallas.com. ESPNDallas.com's Jeff Caplan contributed to this report.